Steam Generator

ABSTRACT

A steam generator of a technical plant, in particular a power plant, comprising a heating gas passage enclosed by a gas-tight enclosing wall, wherein the heating gas passage has a number of heating surfaces through which a flow medium can flow, is to ensure especially reliable cleaning of the heating gas flowing off from the heating gas passage at an especially low design and production cost. To this end, provision is made according to the invention for at least one of the heating surfaces to be at least partly provided with a catalytic coating on its side facing the heating gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2007/050030, filed Jan. 3, 2007 and claims the benefitthereof. The International Application claims the benefits of Europeanapplication No. 06003189.5 filed Feb. 16, 2006, both of the applicationsare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a steam generator of a technical plant, inparticular a power plant, comprising a heating gas passage enclosed by agas-tight encasing wall, wherein the heating gas passage has a number ofheating surfaces through which a medium can flow.

BACKGROUND OF THE INVENTION

In a power plant with a steam generator, the heating gas generated inthe burner during the combustion of a fossil fuel or the hot exhaust gasflowing from a gas turbine is used in the steam generator to vaporize aflow medium. The steam generator normally has steam generator pipesbundled or assembled to form heating surfaces for the vaporization ofthe flow medium, whose heating leads to a vaporization of the flowmedium within the heating surfaces by the radiated heat of the burnerflames or the convective interaction with the heating gas. In this case,a part of the heating surfaces generally directly forms the gas-tightenclosing wall of the heating gas passage, also referred to as the gasflue, with another part of the heating surfaces being connected to theheating gas passage and projecting into said passage to increase theeffective useable surface. The steam produced by the steam generator canin turn, for example, be provided for a connected external process or todrive a steam turbine. If the steam drives a steam turbine, a generatoror a working machine is normally driven by the turbine shaft of thesteam turbine. If it is a generator, the power generated by thegenerator can be used to supply an interconnected and/or isolatedsystem.

Depending on the type of fuel used and on the design characteristics ofthe steam generator, the exhaust gas leaving the steam generator or thedownstream technical system contains pollutants in the form of nitrogenoxides, carbon oxides and/or sulfur oxides, as well as solid particlessuch as flue ash and/or soot, which can damage the environment. Inmodern power plants, efforts are increasingly made to minimize thepollutant emissions by primary measures, as they are called, whichmainly concern the burners of the steam boiler or the gas turbineupstream of the boiler, and are designed to result in optimized,low-emission combustion processes. In cases in which such measures arenot possible or are associated with expensive conversion measures, orare not adequate in order to comply with the legally specified limitvalues, the so-called secondary measures are necessary in order tofilter and separate or otherwise render the pollutants in the flue gasor exhaust gas harmless, e.g. by chemical conversion into less harmfulreaction products or products which are easier to handle.

DeNO_(x) catalytic converter devices, which are fitted at a suitablepoint in the heating gas passage of the steam generator by means ofassociated supporting structures, are normally used, particularly toreduce the nitrogen oxides present in the heating gas, see example in EP0 753 701 A1. In DeNO_(x) catalytic converter devices of this kind,nitrogen oxides contained in the heating gas flowing through are reducedby spraying in, or injecting in, an ammonia solution which reacts withthe catalytic material, with water (H₂O) and elementary nitrogen (N₂)being produced as reduction products. The process is also known asselective catalytic reduction (SCR). The disadvantage with this conceptis that the DeNO_(x) catalytic converter device requires additionalinstallation space within the heating gas passage and comparativelyexpensive mounting and attaching structures, which increases the totalcost for the erection and installation of the steam boiler. Because ofthe absence of installation space, existing old systems can frequentlyonly be retrofitted and brought up to date at a comparatively high cost.

SUMMARY OF INVENTION

The object of the invention is therefore to provide a steam generator ofthe type mentioned in the introduction, which has a particularly lowdesign and production cost and guarantees especially reliable cleaningof the heating gas before said gas leaves the steam generator at theoutlet end.

The object is achieved according to the invention in that at least oneof the heating surfaces is at least partially provided with a catalyticcoating on its side facing the heating gas.

The invention is based on the concept that a steam generator designedfor a particularly low design and production cost should have anespecially low overall height, or particularly short overall length ifthe boiler is of the horizontal type, so that the material requirementand the time expenditure for the production of the enclosing walls, andwhere necessary, the static requirements for the associated supportingstructure are minimized. A particularly compact and simple constructioncan be achieved in that the exhaust gas cleaning devices, which werepreviously provided as separate components requiring a comparativelyhigh installation space, can be integrated into the heat transferelements, already present in any case and required to operate the steamgenerator, especially into the heating elements and heating surfacesformed from the steam generator pipes. These are especially evaporatorheating surfaces or economizer heating surfaces in an area of the steamgenerator in which the heating gas flowing past usually is attemperatures of approximately 300° C. to 400° C. A particularlyeffective and space-saving integration of the cleaning function for theheating gas can be achieved with an increased cleaning effect at thesame time, in that the heating surfaces are used not only for heattransfer but also additionally as carriers for a catalytically activesurface coating. In this case, the coating is advantageously chosen sothat by contact and interaction with the through-flowing heating gas itcauses, or at least promotes, a decomposition or conversion ofpollutants carried in the heating gas, without it being itself“consumed” or wearing out (catalytic converter principle). With theconcept envisaged here, the supporting structures for the previouslynormal separate catalytic converter devices in particular are thusomitted.

With an application of this concept which is particularly important inpractice, the surface coating applied to the heating surfaces of thesteam generator is advantageously such that in conjunction with thealready known and proven principle of selective catalytic reduction(SCR), which is also normally used in the previous DeNO_(x) catalyticconverter devices and denitrogenation systems, it brings about aconversion or decomposition of the nitrogen oxides and/or carbon oxidespresent in the heating gas. Furthermore, for this purpose, an injectiondevice for a reduction means, especially a liquid containing ammonia, isadvantageously arranged in the heating gas passage of the steamgenerator so that when the steam generator is operating the heating gasdecomposed by the injection of the reduction means flows over therespective catalytic coating surface.

In other words, the catalytically active coating on the heatingsurface(s) or on the surface of the steam generator pipes forming theparticular heating surface serves to activate and/or maintain a reactionbetween the reducing agent introduced into the heating gas and thenitrogen oxides of the heating gas. With the SCR process, the nitrogenoxides are reduced to nitrogen and water by the presence and interactionof the catalytic converter material, with the aid of the reducing agentinjected into the gas flue or heating gas passage, usually with air asthe carrier current.

The amount of nitrogen oxide occurring in the steam generator normallydepends on the type of fossil fuel burned and the way in which the steamgenerator operates. To be able to comply with the legal limits at alloperating states, the amount of reducing agent injected is thereforevaried depending upon the fossil fuel used and the operating parametersat that moment.

Particularly effective catalytic coatings can be achieved on the basisof recent knowledge from nanostructure research. A design aim achievablewith the aid of nanotechnology is, in particular, coating material thatcan be easily and durably applied to almost any surface contours of thesteam generator heating surfaces. Materials particularly used as thecatalytic materials for the catalysis of the nitrogen oxidedecomposition are titanium oxide, vanadium pentoxide or tungsten oxide.Catalytic converters based on zeolite can be used as an alternative.Materials particularly preferred for this are ammonium mordenite andH-beta-zeolite. Finally, it is also conceivable that catalytic convertermaterials will in future be discovered or developed which also activateor promote a decomposition of pollutants (e.g. nitrogen oxides) carriedin hot gas, without the injection of a reducing agent or other chemicalreagent. In this case, the injection device described above can also beomitted.

The catalytic coated heating surface(s) can be part areas of theenclosing wall of the wall heating surfaces forming the heating gaspassage. Additionally or alternatively, heating surfaces projecting intothe heating gas passage or other built-in parts with a catalytic surfacecoating of the aforementioned type can be provided. Platen heatingsurfaces, as they are called, are particularly suitable for thispurpose. A platen heating surface in this case is a number of steamgenerator pipes connected in parallel for the flow of the flowingmedium, terminating in a common inlet and a common outlet collector,with the steam generator pipes lying close together in a plane and thusforming a number of plate-type heating surfaces mounted within the gasflue. Alternatively, the coated heating surface can also be a pipe nestheating surface with which the individual pipes are not, in contrast toa platen heating surface, joined to each other by webs. Especially withthe inclusion of such internal heating surfaces, the overall surfaceavailable for catalytic coating is comparatively large. Previouslyunattainable reduction rates for the pollutants carried in the heatinggas can be achieved in this way and therefore also particularly lowpollutant limits can be reliably and permanently complied with out theneed for design compromises, such as unsatisfactory temperatureprofiles, allowing for flow instability, expensive firing concepts andburner configurations etc, which would increase the production cost orimpair the energy efficiency of the steam generator.

For a particularly effective exhaust gas cleaning, the catalytic coatedheating surfaces should be arranged in an area of the heating gaspassage which, with regard to the usual operating temperaturesprevailing in said heating gas passage, guarantees a high effectivenessof the generally temperature-sensitive catalytic cleaning process. Inthe case of denitrogenation of the heating gas according to the SCRprinciple, the respective catalytic coated heating surface is thereforeadvantageously arranged in a region in which the heating gas flowingpast has a temperature of between approximately 300° C. and 400° C. atrated load operation of the steam generator.

The concept explained here can be applied to steam generators ofdifferent construction and operating principle, e.g. with horizontal orupright boilers as well as with natural circulation, forced circulationor forced throughput, and with different firing concepts, e.g. fluidizedbed firing or dry dust firing. Also, the arrangement of the heatingsurfaces at the flow medium end, for example evaporator heatingsurfaces, superheater heating surfaces and also heating surfaces formingpart of an economizer or air preheater can be specified in advancevirtually as required.

In a particularly preferred first variant, which, for example, is alsoparticularly suitable for a waste-heat steam generator arrangeddownstream of a gas turbine, the steam generator includes a heating gaspassage, also known as a horizontal gas passage, through which heatinggas or exhaust gas from the gas turbine flows in an essentiallyhorizontal direction, provided with a number of heating surfaces eachprovided with a catalytic coating. This can especially be evaporatorheating surfaces or economizer heating surfaces in an area of the steamgenerator in which the heating gas flowing past normally hastemperatures of approximately 300° C. to 400° C.

In a second, also particularly advantageous, variant which isparticularly useful for fossil firing by means of burners fitted in thesteam generator, the steam generator is constructed with an uprightboiler as a two-passage type and during operation has a first verticalgas flue through which heating gas flows from bottom to top but which atthe heating gas end is connected via a horizontal gas flue to a secondvertical gas flue through which the heating gas flows from top tobottom. In this case, the particular catalytic coated heating surface ispreferably arranged in the area of the horizontal gas flue or of thesecond vertical gas flue or also in a section of the heating gas passagedownstream of this heating gas end, whereby it can preferably be asuperheater heating surface, an economizing heating surface or also anair preheating surface depending on the design operating temperature ofthe catalytic converter material.

The particular advantages achieved by the invention are that by theapplication of a catalytically active coating, designed for flue gascleaning and denitrogenation, to the heating surfaces present in a steamgenerator, the DeNO_(x) catalytic converter device, which has been usualup to now and required a separate installation space, can be omitted,which enables a particularly compact and cost-effective construction ofthe steam generator. Because the heating surfaces available for coatingare comparatively large, very good pollution reduction rates can beachieved even where the requirements regarding the burner(s) are keptlow. Furthermore, existing power plants or steam generators, in which upto now no secondary emission reduction measures have been taken, can berelatively easily retrofitted and adapted to the increasingenvironmental requirements and more stringent legal limits, e.g. by theretrospective application of a catalytically active coating to theexisting heating elements or by replacing uncoated heating surfaces bycoated heated surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the invention are further explainedwith the aid of drawings. The drawings are as follows:

FIG. 1 A schematic showing a side elevation of a two-flue type generatorfired by fossil fuel,

FIG. 2 A schematic showing a side elevation of a waste-heat steamgenerator with a horizontal steam boiler,

FIG. 3 A plan view of a heating surface of a steam generator formed bysteam generator pipes.

Parts that are the same in all illustrations are given the samereference characters.

DETAILED DESCRIPTION OF INVENTION

The steam generator 2 according to FIG. 1 designed as a once-throughsteam generator includes a number of burners 4 for a fossil fuel. Theburners 4 are arranged in a combustion chamber 6 which is formed by abottom part of the enclosing wall 8 of a first vertical gas flue 10.This enclosing wall 8 merges at the bottom end of the vertical gas flue10, formed by said enclosing wall 8, into a funnel-shaped base 12. Thesteam generator 2 is of the two-flue type. For this purpose a secondvertical gas flue 16 is arranged, via a horizontal gas flue 14,downstream of the first vertical gas flue 10 for the heating gasgenerated by burning the fossil fuel. A further horizontal gas flue 18,which finally terminates in an exhaust stack or chimney (notillustrated) is connected to the second vertical gas flue 16. The gasflues 10, 14, 16, 18, together form a heating gas passage 20. Thecomplete arrangement, apart from the exhaust stack, is arranged inside asupporting structure 22 and supported by said supporting structure 22 bymeans of struts.

The enclosing wall 8 of the first vertical gas flue 10 is constructed ofsteam generator pipes (not illustrated in more detail) which are weldedto each other on their long sides by means of webs or “fins” to form agas-tight joint, and which wind, essentially in the form of a helix,around the cylindrical inner space 24. A number of adjacent steamgenerator pipes are thus assembled together to form an evaporatorheating surface 26, forming a segment of the enclosing wall 8, for aparallel admission of water as the flow medium. Water preheated by aneconomizer 28 is applied via a common inlet manifold (not illustrated)to the inlet ends of the steam generator pipes forming an evaporatorheating surface 26. The water vapor, generated in the steam generatorpipes of an evaporator heating surface 26 due to the heating by theburners, flows from the output end via a common outlet manifold (notillustrated) and is then fed to a superheater unit.

For this purpose, a number of superheating surfaces 30 in the form ofplaten heating surfaces are arranged downstream of the evaporatorheating surfaces 26 of the first vertical gas flue 10, with the platenheating surfaces being arranged mainly in the area of the horizontal gasflue 14. Each of the, mainly convectively-heated, superheater heatingsurfaces 30 has a number of parallel steam generator pipes for thethroughflow of the flow medium. The steam generator pipes of asuperheater heating surface 30 are connected together to form adiaphragm wall. To achieve this, each steam generator pipe of therespective superheater heating surface 30 is welded by a web in eachcase with each adjacent steam generator pipe of the same superheaterheating surface 30. The steam generator pipes assigned in each case to asuperheater heating surface 30 are arranged close to each otherhorizontally in a plane and each thus, as a platen heating surface,forms a plate-type heating surface. The plate-type superheater heatingsurfaces 30 formed in this way are mounted inside the horizontal gasflue 14. The superheated steam, above the evaporation temperature,flowing from the steam generator pipes of the superheater heatingsurfaces 30 can, for example, be used to drive a steam turbine, notillustrated here.

A part of the superheater heating surfaces 30 can also be used for theintermediate superheating of the partially expanded flow medium flowingfrom the first turbine stage of the steam turbine, so that the flowmedium, which is then reheated, can be fed to the next stage of thesteam turbine.

Due to the heat transfer to the flow medium flowing through theevaporator heating surfaces 26 and the superheater heating surfaces 30,the temperature of the heating gas flowing in the heating gas passage 20increases as it progresses along the flow path. When entering the secondvertical gas flue 16, the temperature of the heating gas is stillapproximately 300° C. to 400° C. With this amount of residual heat, theflow medium, which is still cold and liquid, flowing through the pipesof the economizer 28, also called a feed-water preheater, is preheatedbefore entering the steam generator pipes of the evaporator heatingsurfaces 26 downstream of the economizer at the flow medium end. Thistype of utilization of the heating gas waste heat enables the overallefficiency of the steam generator to be increased by a few percentagepoints. The economizer 28 has several pipe-bundle heating surfaces eachmade up of pipes 32 arranged in parallel at the flow medium end, theeconomizer heating surfaces 34, which project into the heating gaspassage 20. The base surfaces of the plate-type economizer heatingsurfaces 34 are in this case aligned parallel to the direction of flowof the heating gas, so that both sides of the pipe arrangement can beexposed to the flow. The individual pipes 32 themselves are arrangedvertically relative to the direction of flow of the heating of gas inthe embodiment shown in FIG. 1.

After passing the economizer heating surfaces 34, the temperature of theheating gas is typically still only about 250° C. to 400° C., which is,however, sufficient for a convective heating of the air preheater 36arranged in the end area of the heating gas passage 20. Similar to theeconomizer 28, the air preheater 36 has heating surfaces 38 formed frompipe bundles, but it is not the flow medium to be vaporized which flowsthrough these heating surfaces 38 but instead the combustion air to befed to the burners 4 of the steam generator 2, which means that they arepreheated before entering the combustion zone.

With its compact and simple construction, the steam generator 2 isdesigned for an effective cleaning and denitrogenation of the heatinggas flowing from the heating gas passage 20 as exhaust gas. For thispurpose, as shown in the side elevation in FIG. 3, the heating surfaces34 of the economizer 28, i.e. those pipes 40 carrying the flow medium,are provided on the outside facing toward the heating gas with a coating44, as a catalytic converter, which effects the activation andmaintenance of an SCR denitrogenation reaction. In this case, e.g.titanium oxide or zeolite material is used as the coating material,which is applied to the base material of the pipes 40 and/or of any websconnecting them by means of a coating process familiar to the personskilled in the art, before assembling the steam generator 2. By means ofthe catalytic converter material, the activation energy required for theSCR reaction, during which the nitrogen oxide carried in the heating gasis reduced to elementary nitrogen and water by an ammonia solutioninjected into the heating gas flow, is reduced.

The ammonia injection takes place, as shown in FIG. 1, with the aid ofan injection device 46 arranged in the heating gas passage 20 upstreamof the economizer heating surface 34, with the injection device 46 beingfed by means of a compressed air device (not illustrated) from a storagetank for ammonia water. The nozzles of the injection device 46 areadjusted and aligned so that the best possible mixture of theammonia-laden liquid spray with the heating gas is obtained combinedwith the best possible uniform wetting of the catalytic coatedeconomizer heating surfaces 34 over which the created mixture flows.

With a further embodiment not shown here, instead of the economizerheating surfaces 34, the heating surfaces 38 of the air preheater 36,are provided with the catalytic coating. In this case the injectiondevice 46 is arranged in the section of the heating gas passage 20between the economizer 38 and the air preheater 36. Depending on theoperating range of the catalytic converter material and the temperatureprofile, determined by the heating, along the flow path for the heatinggas, it can be useful, instead of the economizer 28 or the air preheater36, to apply the catalytic coating to the superheater heating surfaces30 arranged in the horizontal gas flue 14.

FIG. 2 shows a further embodiment of a steam generator 2′ designed as awaste-heat steam generator, with a horizontal water-pipe boiler, whichis arranged downstream of a gas turbine (not illustrated here) and isheated by the exhaust gas from the gas turbine. The exhaust gas of thegas turbine in this case flows through the horizontal gas flue 48,enclosed by the gastight enclosing wall 8′, in the direction shown bythe direction arrow 50. In doing so, the heating gas loses a large partof the heat it contains by convective heat transfer to the wall heatingsurfaces forming the enclosure wall 8′ or to the pipe bundle heatingsurfaces arranged inside the heating gas passage 20′, as a result ofwhich the flow medium carried in the heating surfaces is preheated,vaporized and then superheated. For this purpose economizer heatingsurfaces 34′, evaporator heating surfaces 26′ and superheater heatingsurfaces 30′, arranged appropriately in series at the flow medium end,are provided, with in the exemplary embodiment shown in FIG. 2 theevaporator heating surfaces 26′ being further subdivided into theheating surfaces of a medium pressure evaporator 52 and of a highpressure evaporator 54. The heating gas whose temperature has reducedthe most after its heat output to the flow medium leaves the steamgenerator 2′ via an exhaust stack 56 designed as a vertical gas flue.Needless to say, many variations with respect to the configuration andflow medium arrangement of the heating surfaces are familiar to theperson skilled in the art, but are not dealt with individually here.

Furthermore, what is decisive is that a number of heating surfaces areprovided at least partially with a catalytic surface coating 44 on theirside facing the heating gas, which brings about or promotes a reductionin the pollutant carried in the heating gas. When choosing which heatingsurfaces are to be coated, the temperature limits of the local patternof the heating gas temperature (in steady-state, rated-load operation)are again an important design criterion depending on the temperaturelimits to be maintained for the catalytic reaction. The heating surfaceconfiguration shown in FIG. 2 especially considers the heating surfaces26′ of the medium pressure evaporator 52 and of the high pressureevaporator 54, and also the economizer heating surfaces 34′, and in FIG.2 the economizer heating surfaces 34′ were chosen, by way of example,for the coating in each case. The catalytic coating 44 is schematicallyshown in FIG. 2 by the hatching. Similar to the exemplary embodimentshown in FIG. 1, with the steam generator 2′ in FIG. 2 an injectiondevice for the chemical reagent to be injected into the heating gas canbe arranged upstream of the coated heating surfaces in the heating gaspassage 20′. For clarity however, it is not shown in FIG. 2.

1.-10. (canceled)
 11. A steam generator of a power plant, having aheating gas passage enclosed by a gas-tight enclosing wall, comprising:a plurality of heating surfaces through which a flow medium flows,wherein the heating surfaces are evaporator heating surfaces oreconomizer heating surfaces assigned to an economizer; and a catalyticcoating arranged on a face of at least one of the heating surfacesfacing the heating gas.
 12. The steam generator as claimed in claim 11,wherein the coating applied to the respective heating surface catalyzesor brings about a conversion or decomposition of pollutants present inthe heating gas.
 13. The steam generator as claimed in claim 12, whereinthe coating applied to the respective heating surface catalyzes orbrings about a conversion or decomposition of nitrogen oxides and/orcarbon oxides.
 14. The steam generator as claimed in claim 13, furthercomprising an injection device for a reduction agent arranged such thatduring the operation of the steam generator the heating gas, decomposedfollowing the injection with the reduction agent, flows over therespective catalytic coated heating surface.
 15. The steam generator asclaimed in claim 14, wherein the reduction agent is a liquid containingammonia.
 16. The steam generator as claimed in claim 15, wherein thecatalytic surface coating is formed by nanoparticles applied to the basematerial of the respective heating surface, the catalytic surfacecoating material is selected from the group consisting of: titaniumoxide, vanadium pentoxide, tungsten oxide, a zeolite material andcombinations thereof.
 17. The steam generator as claimed in claim 16,wherein the respective catalytic coated heating surface is a wallheating surface integrated into the enclosing wall.
 18. The steamgenerator as claimed in claim 17, wherein the respective catalyticcoated heating surface is a platen heating surface, arranged within orprojecting into the heating gas passage.
 19. The steam generator asclaimed in claim 18, wherein the respective catalytic coated heatingsurface is arranged in an area in which the heating gas flowing past hasa temperature of between approximately 3000 C and 4000 C at rated loadoperation of the steam generator.
 20. The steam generator as claimed inclaim 19, wherein the steam generator is a horizontal type steamgenerator.
 21. The steam generator as claimed in claim 20, wherein thehorizontal type steam generator is a waste-heat steam generator with ahorizontal gas passage through which the heating gas flows in anessentially horizontal direction.
 22. The steam generator as claimed inclaim 19, wherein the steam generator is of the two-passage type withfossil fuel firing, with a first vertical gas passage through which theheating gas flows from bottom to top during operation, the heating gasend connected via a horizontal gas passage to a second vertical gaspassage through which the heating gas flows from top to bottom, therespective catalytic coated heating surface is a superheater heatingsurface, or an economizer heating surface, arranged in the area of thesecond vertical gas passage.
 23. The steam generator as claimed in claim22, wherein the respective catalytic coated heating surface is part ofan air preheater arranged at an outlet end in the heating gas passage.